1. Field of the Invention
The present invention relates in general to a resinous circuit board having a substrate made of resin or a composite material containing resin, and more particularly to a resinous circuit board provided with pins having a high joining strength.
2. Description of the Related Art
As circuit boards for mounting thereon electronic devices, there are heretofore known a ceramic circuit board made of an insulating ceramic material such as alumina and a resinous circuit board made of an insulating resinous material such as epoxy resin and a composite material consisting of resin and glass fiber. In case of a ceramic circuit board having pins serving as input and output terminals thereof, the pins are attached to a substrate by bringing the heads thereof in the form of a nailhead, into contact with pin pads disposed at a front or rear main surface of a substrate and then soldering the heads to the pin pads.
In case of a resinous circuit board substrate having pins serving as input and output terminals thereof, it is considered, as shown in FIG. 6, to similarly solder the pins P1 having heads P1a in the form of a nailhead to pin pads PD provided to a substrate 1. However, the pins P1 are low in the joining strength and lack reliability, so they are not fit for practical use. For this reason, in order to increase the joining strength, it has been practiced, as shown in FIG. 7A, to first form through holes TH extending between the upper and lower surfaces (i.e., in the thickness direction) of the substrate 1, then press fit the pins P2 having heads in the form of a nailhead in the through holes TH and fixedly attach the pins P2 to the substrate 1 by using solder SR, or as shown in FIGS. 7B and 7C to first form through holes TH and bottomed holes H in the respective substrates 1, press fit pins P3 and P4 having flanges P3a and P4a at a portion intermediate between the opposite longitudinal ends, into the respective holes TH and H, and fixedly attach the pins P3 and P4 to the substrates 1 by using solder SR.
However, as will be understood from FIGS. 7A to 7C, such an arrangement of pins P2, P3 and P4 requires a number of large holes TH and H of the diameter nearly equal to the pins P2, P3 and P4 to be formed in the substrates 1, thus decreasing the area at the front or rear surface for arrangement of wiring conductors (not shown) and therefore considerably lowering the freedom in design of the wiring conductors. For this reason, a complicated wiring requires a increased number of insulating layers to be laminated. Further, the cost for machining the holes is necessitated. Thus, the prior art resinous circuit board is costly.
It is accordingly an object of the present invention to provide a resinous circuit board which has pins joined to its substrate with a high joining strength, while having a high freedom in design of wiring and being of low cost.
To achieve the above object, there is provided according to an aspect of the present invention a circuit device comprising a circuitized substrate made of a resinous material, a plurality of pin pads formed on a surface of the substrate, and a plurality of pins each having a pin main body and an abutment section bulging out from an end of the pin main body, each of the pins being soldered at the abutment section to each of the pin pads, wherein the abutment section is made of solder higher in melting point than solder used for soldering each of the pins to each of the pin pads.
According to another aspect of the present invention, there is provided a circuit board comprising a circuitized substrate made of a resinous material, a plurality of pin pads formed on a surface of the substrate, and a plurality of pins each having a tip end portion and a head portion, each of the pins being soldered to each of the pin pads in such a mariner as to contact at the head portion to each of the pin pads, wherein the head portion includes a flange section integral with the tip end portion and an abutment section bulging out from the flange section in the direction opposite to the tip end portion, the abutment section being made of solder higher in melting point than solder used for soldering each of the pins to each of the pin pads.
As described above, in case a pin made of kovar and having a head in the form of a nailhead is soldered to a pin pad on a resinous substrate similarly to a prior art ceramic circuit board, the joining strength of the pin is low. This is considered due to stress concentration at the joint between the head of the pin and an insulating substrate made of resin or a composite material containing resin. In contrast to this, in the above structure of this invention, the pin is soldered to the pin pad by interposing the abutment section between the flange section of the pin and the pin pad in such a manner as to allow the abutment section to bulge out from the flange section in the direction opposite to the tip end portion. Thus, soft solder or hard solder which is softer than a pin main body made of kovar, 42Ni-Fe, copper or the like, is present between the flange section of the pin and the pin pad. For this reason, the stress caused at the joint between the pin and the pin pad is absorbed by deformation of the soft solder or hard solder and thereby released, so the joint between them is hard to break. Therefore, the above structure of this invention enables to attain a higher joining stress between the pin and the pin pad as compared with the case the pin is directly soldered at the head to the pin pad.
The circuitized substrate includes wiring conductors disposed at least on the front surface, on the rear surface or within thereof, and can be comprised of a single insulating layer or a plurality of insulating layers. The resinous material for the substrate can be resin such as epoxy resin, polyimide resin, BT resin and PPE resin. The resinous material also can be a composite material consisting of organic fibers such as glass fibers and polyester fibers, impregnated with resin, e.g., glass-epoxy resin and glass-BT resin.
The soft solder or hard solder constituting the abutment section is selected from those having a higher melting point than the solder used for soldering the pin to the pin pad, i.e., selected from silver solder such as Agxe2x80x94Cu alloy, gold solder such as Auxe2x80x94Si alloy, Auxe2x80x94Sn alloy and Auxe2x80x94Ge alloy, and high temperature soft solder such as 95Pbxe2x80x945Sn and 90Pbxe2x80x9410Sn.
Further, the solder for soldering the pin to the pin pad is determined on consideration of the soldering temperature of an electronic device or devices to be mounted on the circuit board later, e.g., can be selected from Pbxe2x80x94Sn solder such as 37Pb-73Sn eutectic solder, 50Pb-50Sn and 90Pb-10Sn and Snxe2x80x94Ag solder such as 96.5Sn-3.5Ag.
In case the abutment section having a part-spherical or other bulged shape is made of the same metal as the pin main body, it is a general practice to form the abutment section together with the pin main body by pressing simultaneously so that the abutment section and the pin main body constitutes a single piece. However, since the pin is so small, formation of such a spherical abutment section is quite difficult and the cost per each piece is so high. Generally, the number of pins used in the circuit board is so large, i.e., ranges from hundreds to about one thousand, so a little increase of the cost per piece causes a large increase of the cost of the unit. In this connection, the pin used in this embodiment has the part-spherical abutment section made of solder and covering the side surface of flange section in the form of a nailhead. Thus, the pin main body can have an end in the form of a nailhead and therefore can be formed with ease. Furthermore, by placing a piece of solder on the side surface of the flange section and heating and melting it, the abutment section can be formed into a part-spherical or spherical shape and can be fused to the flange section with ease.